1. Field of the Invention
The present invention relates to an input/output circuit for a communication device, such as a network control unit (NCU) of a facsimile machine, a message reception/transmission circuit of a telephone set.
2. Description of the Related Art
Input/output circuits used in a communication devices, such as a facsimile machine, a telephone set, are provided with a number of amplification circuits for amplifying input signals received through a communication line and/or output signals sent out to the communication line. Due to the fact that the standards of the communication devices are not the same depending on a place (the term "place" encompasses a country, state, territory or local area in the same country, same state, same territory), it has been required to adjust an amplification factor of each amplifier circuit provided in the communication device.
To this end, the input/output circuit of the communication device includes a plurality of negative feedback amplifiers. As shown in FIG. 3, the negative feedback amplifier is typically configured by an operational amplifier OP31, and resistors R31, R32 and R33. To change the amplification factor of this amplifier, the resistance values of the resistors R31 and R32 are changed. The resistor R31 is a feedback resistor connected across the inverting input terminal and the output terminal of the operational amplifier OP31. The resistor R32 is an input resistor connected to the negative input terminal of the operational amplifier OP31. To the non-inverting input of the operational amplifier, a reference voltage REF is applied through the resistor R33.